Emerging MRI-based quantitative imaging biomarkers for fat and iron deposition are based on chemical shift encoded techniques. In these techniques, multiple images are acquired at increasing echo times, and quantification of fat and iron is performed by parametric fitting of the acquired data (e.g., to measure proton-density fat-fraction and R2*, which are biomarkers for tissue triglyceride and iron deposition, respectively).
Often, images are acquired using MRI pulse sequences that include bipolar readout gradients, where consecutive echo times are obtained with opposite gradient readout polarities. Bipolar acquisitions are very common because they allow more efficient use of acquisition time compared to monopolar pulse sequences, where all the echoes are acquired with the same gradient readout polarity (e.g., by inserting “flyback” gradients in between). Unfortunately, bipolar pulse sequences result in phase errors between the images obtained with different gradient polarities (i.e., even versus odd echoes). These phase errors can result in large errors in the quantification of fat and iron.
Specialized pulse sequences can be used to correct for these phase errors. In these specialized pulse sequences, additional calibration data are acquired, which enables calibration and correction of phase errors. However, these specialized pulse sequences are not widely available, partly due to their relatively high cost. Therefore, many sites only have standard pulse sequences, which suffer from phase errors, which complicates the widespread application of emerging fat and iron quantification techniques.
Thus, there remains a need to provide methods for quantitative chemical shift encoded MRI that automatically corrects for the phase errors associated with bipolar readout gradients, or that otherwise produces quantitative maps not affected by such errors.